The 54-year-old Japanese man teamed up with Alexander Yee, a U.S. computer science student, to set out to try and calculate the constant that has fascinated mathematicians for millennia.

Pi is the ratio of a circle's circumference to its diameter. It starts with 3.14159 in a string of digits that are believed to never repeat or end.

"Alexander provided software and I was in charge of hardware. We couldn't have achieved the results without either of us," Kondo said, adding that the two men worked together while communicating by email.

It took 90 days to calculate pi at Kondo's home by using a desktop computer with 20 external hard disks. The computer ran Windows Server 2008R2 operating system and used Intel microprocessors. Verification took 64 hours.

Kondo built the computer himself by using parts bought from local electronics shops and Internet stores.

"I don't really want to say how much it cost me as my family may hear it... it's about 18,000 dollars," he told AFP by telephone.

He said that the computer reached five trillion decimal places at midnight in Japan. "I was alone in the room at the moment... I know this is nothing but self satisfaction," he told AFP.

He added that his mother and wife who live with him were sleeping at that time and later showed "no particular feelings" about the accomplishment.

Fabrice Bellard of France said earlier this year that he had used an inexpensive desktop computer to calculate pi to nearly 2.7 trillion decimal places. That was 123 billion digits more than the previous record set by Japanese professor Daisuke Takahashi last August.